Forming composite articles comprising titanium or titanium-base alloys and aluminum and aluminum-base alloys



Dean K. Haninlr, Birmingham, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Application May 10, 1952,

Serial No. 287,246

7 Claims. (Cl. 22-204) This invention relates to forming composite articles by casting and bonding aluminum or aluminum-base alloys onto titanium or titanium-base alloys.

The principal object of my invention is'to provide a simple, practical and inexpensive method for strongly bonding aluminum or aluminum-base alloys to titanium or titanium-base alloys. The process results in forming a' composite article of titanium and aluminum parts in which there is no brittleness at the junction between the two metals. Such a process has many useful applications, such as forming aluminum bearings having titanium backings.

Other objects and advantages of my invention will more fully appear from the following detailed description of a preferred process for forming composite aluminum and titanium articles.

The aluminum or aluminum-base alloy is strongly bonded to the titanium or titanium-base alloy, in accordance with my invention, by an intermediate phase of aluminum and titanium alloy. This intermediate phase also includes the minor constituents originally present in the titanium or aluminum before they were chemically joined. It will be understood that, by aluminum-base alloys, as hereinafter used, it is meant those aluminum alloys, including pure aluminum, which in general contain at least approximately 80% aluminum. Hence, it will also be understood that the terms aluminum and aluminumbase alloy, as used herein, are interchangeable and are not intended to restrict any phase of the invention to only one of these groups. Similarly, the terminology ititanium-base alloy or titanium-base meta is meant to include pure titanium and all alloys thereof in which titanium constitutes at least 50% of the alloy.

Titanium-base alloys to which aluminum or aluminum base alloys may be bonded in accordance with my invention include the commercially available alloys containing appreciable amounts of chromium. Typical examples of these alloys are those having the following compositions:

' Percent Chromium 3.00 Iron 1.50 Oxygen 0.50 Nitrogen 0.04 Tungsten maxirnum 0.08 Carbon 0.02 Balance substantially all titanium.

. I Percent Qhromium 2.70 Iron 1.30 Oxygen 0.25 Nitrogen 0.02 Carbon 0.02 Tungsten "maximum" 0.04

Balance substantially all titanium.

Similarly, as hereinbefore indicated, my process is adapted for use in bonding pure or commercially PureItitanium to alumi'num and aluminum-base alloys. ;A=typ ical example of this latter type of titanium alloy is one consisting essentially of 0.10% iron, 0.02% nitrogen, carbon not in excess of 0.04%, tungsten not in excess of 0.04%, traces of oxygen, and the balance substantially all titanium.-

In carrying out my invention, the titanium or titaniumbase alloy is immersed in a molten or fused salt bath capable of absorbing oxides of titanium and aluminum. 1 have found that a highly satisfactory salt bath is one comprising, by weight, 37% to 57% potassium chloride (K01), 25% to 45% sodium chloride (NaCl), 8% to 20% cryolite (NaaAlFs), and 0.5% to 12% aluminum fluoride (AlFs). A bath consisting essentially of 47% by weight of potassium chloride, 35% by weight of sodium chloride, 12% by weight of cryolite, and 6% by weight of aluminum fluoride is a specific example of this salt bath which provides excellent results.

While in the foregoing examples the double salt NaaAlFs (cryolite) is included in the bath, it should be understood that an equivalent amount of this component may be supplied in the form of. the single salts, sodium fluoride and aluminum fluoride. However, I have found that it is essential to provide an excess of AlFa over that of the cryolite ratio in order to obtain the desired results.

The salt bath composition usually preferred is one which will become molten when heated to a temperature of approximately 1200" F. or somewhat lower. The above specific salt bath has a melting point of approximately 1180" F. and, if desired, a small amount of lithium chloride may be added to the salt to lower the melting point thereof. For example, the addition of about 20% lithium chloride lowers the melting point of this composition to approximately 1075 F. During operation the temperature of the fused salt bath is maintained somewhat above the melting point of the aluminum or aluminum-base alloy, a temperature above approximately 1200 F. being desirable to obtain effective fluxing action. To avoid excessive volatilization and chemical instability of the molten salt, however, it should not be maintained higher than approximately 1600" F. In general, a salt bath temperature within the range between 1300 F. and 1400 F. is

preferred.

. The surfaces of the titanium-base alloy part are preferably cleaned in any suitable manner prior to the immersion in the fused salt bath. Where the titanium-base al loy has an excessive amount of oil thereon or is badly rusted, more consistent and uniform results may be obtained if mechanical cleaning means, such as grit blasting, sand blasting, hydroblasting or vapor blasting, is 6111- ployed. Other suitable cleaning treatments, such as etching by appropriate fluxes, may be used if desired. This preliminary cleaning treatment, although recommended as apreferred step in the procedure, is not necessary in order. to obtain satisfactory results in most instances. 'Fre" quently no special preparation of the surface of the titanium-base alloy part prior to immersion in the fused salt bath is required since the molten salt: bath flux dissolves titanium oxides, and a small amount of oil will burn oii with no deleterious eilects. The titanium alloy part, of course, must be dry to avoid a steam explosion when immersed in the molten salt.

The fused salt bath must be activated by aluminum in or in contact with it in order to provide effective fiuxing action. This may be accomplished by employing an aluminum or aluminum alloy coated container for the fused salt, or aluminum or an aluminum alloy may be added to the salt. The aluminum or aluminum alloy may be added by immersing a bar or sheet of the metal in the fused salt bath. This bar or sheet of aluminum or aluminum alloy readily melts and settles to the bottom of the bath. t After treatment in the fused saltbath, which serves to;

?atented Mar. 19, 1957 slblz,

clean undesirable oxides from the titanium or titaniumbase alloy and to flux the metal, the titanium-base alloy part is immersed in molten aluminum or molten aluminum-base alloy to form a coating or aluminum or aluminum alloy thereon. in accordance with myv invention, the preferred procedure is to pass an unheated titaniumbase metalpart directly through arelatively thin layer of the fused salt which is floating on a molten aluminum or aluminum-base alloy coating bath. The cold titaniumbase. metal part, in passing through the layer of activated salt fiux,.will become coated with a layer of the which may solidify, but which will remclt after sufficient time of contact with the molten aluminum or aluminum alloy bath underneaththe fused salt layer. An adherent coating of aluminum on the titanium-base metal part may be obtained by this procedure because the aluminum contacts the titanium alloy immediately after the salt is re,- movedfrom the surface, and there is no opportunity for titaniurnoxides to form.

After passing through the fused salt flux and into the molten aluminum or aluminum-base alloy bath, the titanium or titanium-base alloy part is permitted to remain in the coating bath until at least its surface reaches the melting point of the coating metal. The time may vary from as little as about one or two seconds up to several minutes, depending somewhat on the exact procedure used, as will be hereinafter fully explained, and the degree of complication of recesses, etc. in the part being processed.

The titanium-base alloy part with a coating of aluminum or aluminum-base alloy thereon is then preferably withdrawn from the coating bath through the molten salt layer, inserted in located position in a mold and the mold closed. Molten aluminum or aluminum-base alloy is next poured into the mold, preferably while the coating is still in a molten or mushy condition. It is desirable to so design the mold gates as to direct the incoming molten aluminum or aluminum-base alloy up and around the coated titanium-base alloy part to thereby carry any formed oxide films up into the risers of the casting. The

coated titanium-base alloy part should be at or near the temperature of the incoming aluminum or aluminum alloy to avoid chill and blow effects. After cooling, the titanium-base alloy and the aluminum or aluminum-base alloy are strongly bonded into an integral assembly.

The molten metal bath and casting metal may be pure aluminum or aluminum-base alloys and, as hereinbefore indicated, these alloys preferably should contain approximately 80% or more aluminum. An aluminum-base alloy which is satisfactory, especially as the coating metal, is one composed of approximately 7% tin and the balance aluminum. This alloy has a relatively low melting temperature and high fluidity. Effective drainage ofexcess coating metal is obtained because of thishigh fluidity, particularly when the coating is protected from the air by the molten salt flux. Specific examples of other typical aluminum-base alloys which may be employed for either or both the coating metal and casting metal include an alloy composed of about to silicon and the balance aluminum, an alloy composed of approximately 4% copper or 7% tin and the balance aluminum, and an alloy containing 5% to zinc and the balance substantially all aluminum. These specific examples are referred to merely for purposes of illustration and not of limitation.

Best results are obtained when the coating metal bath is'maintained at substantially the same temperature as that of the fused salt bath. At present it is preferred that the temperature of the aluminum or aluminum-base alloy coating bath be maintained at a temperature between about 1250 F. and 1325 F. Temperatures as low as approximately 1150 F. are suitable, however, for the aluminum-base alloys. Pure aluminum melts at approximately 1218 F., and consequently, when employing pure aluminum as the-coating metal, aluminum and salt bath a e alloy p rt.

temperatures of at least 1218" F. must be used. The upper temperature employed with either aluminum-base alloys or pure aluminum aspthe coating metal is approximately 1600" F. A single heating means may be employed, of course, for both the fused salt and molten aluminum baths if they are used in the same container, as hereinbefore described.

The procedure described in detail above constitutes a preferred embodiment of processing in accordance with my invention. However, it is possible to obtain a satis-v factory composite article by departing from the conditions of the preferred embodiment, but the resulting bond joining the subject dissimilar materials without mechanical interlocking must consist of an intermediate alloy interface between the components. For example, modifications in procedure, such as, allowing the aluminum coating to solidify on the titanium surface prior to pouring the aluminum or aluminum alloy, will produce a satisfactory bond between the titanium and aluminum if the heat content of the casting metal is sufficient to rernclt the aluminum coating on the titanium.

ltis, also possible to retain the titanium-base alloy part in the fused salt a sufficient time to heat it to or ahhve the melting point of the aluminum or aluminum-base alloy; Although, in this procedure the titanium-base alloy may be held in the molten salt bath until it reaches the temperature of the salt, it is not necessary to do so; and an adherent coating may be obtained if the titanium alloy part is retained in the salt until it reaches the melting point of the salt. That is, if it is desired to do so, it may be held in the salt for a period of time sutficient to permit h Sa to b come o e the s fac s o h t tanium: base alloy part after the initial solidification of the salt aye thr ee hes at er a ter-native proced re howe e re no adly ot as de irab e a h pr fe e e o e c b d nasmu h a y f d a ea e pp rtunity for oxides to form on the surface of the titanium part. Moreover, the. preferred procedure eliminates the tendency to build up an uneven salt layer which is thicker on large sections and thinner on small sections of the titanium: Although these alternative procedures reduce the length of time it is necessary to hold the titanium-base alloy in the molten aluminum, as compared with the, preferred process hereinbeforedescribed,

they afford no additional advantages over the preferred procedure and require longer holding periods in the salt bath and a deeper layer of the molten salt flux. Hence, it is preferable to immerse the titanium or titanium-base alloy part in the molten aluminum bath immediately after it has contacted the fused salt bath and before the latter has remelted on the titanium alloy part.

It is also possible to obtain satisfactory results by heating the titanium-base alloy the activated fused salt bath until it reaches a temperature above the melting point of the aluminum coating metal and then allowing thetitanium=base alloy and coating of said salt thereon-to coolto some temperature below the melting point of the aluminum or salt before immersion in the aluminumoraluminurn-base alloy coating bath. This procedure is satisfactory so long as the salt coating, if allowed to solidify on the titanium-base alloy part, is not cracked or broken prior to immersion in the molten aluminum or aluminum-base alloy coating bath and so long as the titsniurn-base alloy is permitted to remain in this coating bath for a time and at a temperature sufiicient to reheat the surfaces of the titanium part and the salt coating to a temperature above the melting point of the aluminum coating metal and preferably to at least 1250 F. Satisfactory results may also be obtained by heating the titanium-base alloy in the molten salt bath maintained at a temperature below that of the molten aluminum or alumif rn-base alloy and, while-the salt coating on the titani-urnebase metal part is still molten, immersing the part and salt coating in the coating metal bath and heating the same to a temperature within the range between approximately 1200 F. and-1600 F;

The titanium or titanium-base alloy part may be preheated, if desired, prior to immersion in the fused salt bath. This preheating treatment permits the use of smaller quantities of salt and smaller sized salt bath heating means than are necessary where the titanium alloy part is heated in the molten salt. If the preheating step is employed, it is preferable to heat the metal to be coated under such conditions that the surface thereof is not oxidized; An inert or reducing atmosphere furnace, such as one employing dried and purified helium or argon may be used for this purpose. The preheating temperature is preferably within the range of approximately 1200 F. to 1600 F.

When the titanium or titanium-base alloy part is preheated in a non-oxidizing atmosphere to the temperature of the fused salt bath and is free of oxides of titanium and other foreign matter, the time of immersion in the salt bath, as in the first-mentioned preferred procedure, may be as little as a few seconds if the parts are free of complicated recesses. More complicated shapes may require a longer time in order to insure that the salt thoroughly covers or coats the titanium-base alloy at those portions thereof to which the aluminum or aluminum-base alloy is to be bonded. Where the titanium or titanium-base alloy part has oxides of titanium or other foreign matter on its surface, a longer period of immersion in the salt bath will be required in order to provide clean surfaces. Of course, where the preheating step is not employed, sufiicient time is required for either the fused salt or the aluminum bath to heat the titaniumbase alloy part to a temperature at least as high as, and

preferably somewhat above, the melting point of the aluminum bath material. The exact time will depend on the dimensions of the titanium alloy part and the size and thermal efficiency of the salt bath. Retaining the titanium-base alloy in the fused salt for extended periods of time has no detrimental effects on the resultant product.

The time of immersion in the molten aluminum or aluminum-base alloy bath may vary from as little as one or two seconds up to several minutes, depending on which of the above procedures is selected and on the degree of complication of recesses, etc., in the parts being processed. However, inasmuch as titanium and titaniumbase alloys are generally not readily soluble in aluminum and aluminum-base alloys and do not form a low melting eutectic at the titanium and aluminum interface, they may be safely retained in the molten aluminum for considerable periods of time, if it is found convenient to do so.

If desired, separate containers and heating means may be employed for the salt bath and the coating metal bath, but in all instances the fused salt bath should be activated by aluminum in or in contact with it in order to provide effective fluxing action. Where the fused salt is on top of the molten aluminum or aluminum-base alloy, as described above, the proper activity of the molten salt is automatically obtained. On the other hand, where a separate bath and a separate aluminum or aluminum-base alloy coating bath are employed, it is essential to activate the fused salt by other means in the manner hereinbefore explained. In general, however, it is preferable to have both the fused salt and the molten aluminum coating metal in a single furnace and, after fiuxing in the molten salt bath, to immerse the titaniumbase alloy in the aluminum coating metal beneath the molten salt without transfer through air to a separate pot of molten aluminum. Such a procedure greatly reduces the possibility of having oxides formed on the surface of the titanium alloy part.

The invention has application to forming any composite article consisting of titanium or titanium-base a'l loys to which is bonded a layer of aluminum thicker than can be readily applied by a dipping process.

Various changes and modifications of the embodiments of my invention described herein may be made by those skilled in the art without departing from the principles and scope of my invention as set forth in the appended claims.

I claim:

1. The method of forming a composite article of a metal containing at least 80% aluminum and a metal containing at least approximately 50% titanium, said process comprising immersing a part formed from a metal containing at least approximately 50% titanium in a fused salt bath comprising, by weight, approximately 37% to 57% KCl, to NaCl, 8% to 20% NasAlFe and 0.5% to 12% AlFa, said fused salt being maintained at a temperature between approximately 1200 F. and 1600 F. and activated by aluminum in contact therewith, subsequently immersing said part in a molten bath of a coating metal containing at least approximately 80% aluminum for a period of time sufficient to raise the temperature of surfaces of said part to at least as high as the melting point of said coating metal, removing the coated part from said molten coating metal bath and, before the coating has completely soliditied, casting molten metal containing at least approximately 80% aluminium into contact with said coated part.

2. A method as in claim 1 in which a small proportion not greater than 20% of lithium chloride is added to the fused salt bath.

3. A method of forming a composite article of a titanium-base alloy and a metal consisting of atleast 80% aluminum which comprises immersing the titanium-base alloy in a fused salt bath consisting essentially of 37% to 57% by weight of KCl, 25% to 40% by weight of NaCl, 8% to 20% by weight of NaaAlFs and 0.5% to 12% by weight of AlFs, said fused salt bath being activated by aluminum in contact therewith, said fused salt bath being maintained at a temperature within the range of approximately 1300" F. to 1400 R, thereafter immersing the titanium-base alloy in a molten metal coating bath consisting of at least 80% aluminum maintained at a temperature between about 1250 F. and 1325" B, said titanium-base alloy being retained in said coating bath until surfaces of said alloy reach a temperature at least as high as the melting point of said molten metal removing the coated titanium-base alloy from the coating bath and, before the coating has completely solidified, inserting the coated titanium-base alloy in a mold and casting molten metal consisting of at least 80% aluminum into contact with the coated titanium base alloy.

4. The method of forming a composite article of a titanium-base alloy and an aluminum-base alloy which comprises immersing the titanium-base alloy in a fused salt bath comprising, by weight, approximately 37% to 57% KCl, 25% to NaCl, 8% to 20% NasAlFs, and 0.5% to 12% MP3, said fused salt bath being maintained at a temperature within the range between approximately 1200 F. and 1600 F. and floating on top of a molten aluminum-base alloy coating bath, thereafter lowering the titanium-base alloy into said molten coating metal bath and retaining it therein for a period of time of at least two seconds to form a coating on said titanium-base alloy, removing the coated titaniumbase alloy from the coating bath through the fused salt and, before the coating has completely solidified, inserting the coated titanium-base alloy in a mold and cast ing an aluminum-base alloy into contact with the coated titanium-base alloy.

5. The method of forming a composite metal article which comprises passing a part formed from a metal containing at least titanium into and out of a molten coating. metal bath containing, at. least. 89% aluminum through a molten salt bathfloating on said molten coat ing metal bath, said molten saltrbath Consisting essentially oi? 37% to 5.7% by. weight of KCl, 25% to 45% by Weight of NaCl, 8% to 20% by weight. of NasAlFs and 0.5% to 12%. by weight of A11 3, said molten salt bath being maintained. at a temperature between approximately 1300 F. and 1400 F. While the titanium metal part is passing through, said part being retained in said'coatingmetal bath until at least surfaces of said partreach a temperature above the melting point of said coating metaland, before the coating has completely solidified, casting a molten metal containing at least 80% aluminum into contact with the coated part.

6. A method of forming a composite metal product which comprises passing a metal part of the class consisting of titanium and titanium-base alloys into and out of a molten bath of a coating metal selected from the class consisting of aluminum and aluminum-base alloys through a molten salt layer capable of absorbing titanium and aluminum oxides'fioating on said molten coating metal bath, said part being passed through said salt layer into said coating metal bath sufficiently quickly so that salt which solidifies on surfaces of said part does not remelt untilimrnersed in said coating metal bath, said molten salt layer being at a temperature in excess of 1200 F. While said part is passing therethrough, said part being retained in said coating metal bath until surfaces of said part. reach at least the melting point of said coating metal, and thereafter casting molten metal selected from the class consisting of aluminum and aluminum-base alloys into contact with the coated part before the coating on said part has completely solidified.

7. A method of forming a composite metal product whi h QQI PIises. immer i g a ar icle, formed. o a metal con aining at least pproximates i0 1; titanium n fused salt containing substantial amounts, Q'f allgali metal hlo des. an c p ble of..- bsn hing tit nium n l miu m ox es, an useda ing a j te perature f ppm imate y ,003 t 1,600 El nd p s p of a molten bath of a coating metal containing at least 89% aluminum, said article being retained in said fused salt for an insufficient period of time to raise the tem- 10 ne tu e o s i rticle t the tempe atur of. ai l thereafter lowering said article into said molten coating metal bath, retaining said article in said coating metal bath until surfaces of said article reach a temperature of at least the melting point of said coating metal, sub- 15 sequently remoying the coated article from said coating metal bath through said fused salt, and thereafter cast ing molten metal containing at least 8.0% aluminum into contact with the coated article while the temperature of said "article is near the temperature of the molten cast- 20 nguina References Cited in the file of this patent UNITED TAT PATENTS Chamer Jan. 19. 1954 

1. THE METHOD OF FORMING A COMPOSITE ARTICLE OF A METAL CONTAINING AT LEAST 80% ALUMINUM AND A METAL CONTAINING AT LEAST APPROXIMATELY 50% TITANIUM, SAID PROCESS COMPRISING IMMERSING A PART FORMED FROM A METAL CONTAINING AT LEAST APPROXIMATELY 50% TITANIUM IN A FUSED SALT BATH COMPRISING, BY WEIGHT, APPROXIMATELY 37% TO 57% KCI8 25% TO 40% NACI, 8% TO 20% NA3AIF6 AND 0.5% TO 12% AIF3, SAID FUSED SALT BEING MAINTAINED AT A TEMPERATURE BETWEEN APPROXIMATELY 1200*F. AND 1600*F. AND AQCTIVATED BY ALUMINUM IN CONTACT THEREWITH, SUBSEQUENTLY IMMERSING SAID PART IN A MOLTEN BATH OF A COATING METAL CONTAINING AT LEAST APPROXIMATELY 80% ALUMINUM FOR A PERIOD OF TIME SUFFICIENT TO RAISE THE TEMPERATURE OF SURFACES OF SAID COATING TO AT LEAST AS HIGH AS THE MELTING POINT OF SAID COATING METAL, REMOVING THE COATED PART FROM SAID MOLTEN COATING METAL BATH AND, BEFORE THE COATING HAS COMPLETELY-SOLIDIFIED, CASTING MOLTEN METAL CONTAINING AT LEAST APPROXIMATELY 80% ALUMINUM INTO CONTACT WITH SAID COATED PART. 